It is known that organosilicon compounds that contain acryloxy or methacryloxy groups react with radical-polymerizable monomers such as methylmethacrylate and styrene and are used as starting materials for copolymers obtained from the aforementioned monomers or as modifiers for the aforementioned monomers.
A method known in the art for synthesis of the aforementioned organosilicon compounds that contain acryloxy or methacryloxy groups consists of causing an addition reaction between acryloxy or methacryloy acid esters of phenols or alcohols with aliphatic unsaturated bonds and organosilicon compounds that contain silicon-bonded hydrogen atoms, and then separating the organosilicon compounds with acryloxy or methacryloxy groups from the obtained reaction mixture by distillation. The organosilicon compounds with acryloxy or methacryloxy groups obtained by the above-described method are prone to polymerization and to an increase in the molecular weight that can easily occur during reaction of synthesis or in the step of distillation. For suppressing the undesired polymerization, radical polymerization inhibitors are generally added.
For example, Japanese Laid-Open Patent Application Publication (hereinafter referred to as “Kokai”) Hei 5-186478 (equivalent to EP0520477) discloses a method with the use of N,N-dialkylaminomethylenephenols as polymerization inhibitors. Furthermore, Kokai Hei 9-295987 and Kokai Hei 9-296007 (equivalent to EP0803507), disclose processes with the use of polymerization inhibitors in the form of hindered phenols having an onium-salt structure. These inhibitors are naturally colorless and possess extremely high polymerization-inhibiting power. However, when organosilicon compounds that contain acryloxy or methacryloxy groups undergo distillation in the presence of the aforementioned polymerization inhibitors, the distillation fractions are subject to coloration, and it becomes very difficult to obtain colorless distillation fractions of high purity.
Kokai Sho 48-85501 (equivalent to U.S. Pat. No. 3,801,615) discloses a method for decoloration of benzoquinone, which is a coloring component, by adding an olefinic unsaturated conjugated diene to oxidation-polymerizable monoolefinic unsaturated monomer synthesized by distillation with the use of hydroquinone as an inhibitor. However, the use of hindered phenols in the above method as polymerization inhibitors cannot prevent coloration of the distillation fractions. this occurs, probably, because the benzoquinone and the coloring substance admixed with the distillation fraction have different structures.
It is an object of the present invention to provide a method for inhibiting coloration of products of distillation of organosilicon compounds with acryloxy or methacryloxy groups when the distillation is carried in the presence of polymerization inhibitors in the form of hindered phenols having an onium-salt structure or their hindered phenol precursors.